The present invention relates to the field of substrates, especially for electronic or optical components. More specifically, it relates to a process for preparing substrates, especially semiconductor substrates, which will be used for producing these components, or else to a process contributing to the production of these components themselves on a substrate.
Document U.S. Pat. No. 5,374,564 by M. Bruel already discloses a process for fabricating thin films of semiconductor materials which comprises the exposure of a wafer of semiconductor material
to a step of bombarding one face of the substrate with ions so as to implant these ions in sufficient concentration to create a layer of microcavities,
to a step of bringing this face of the substrate into intimate contact with a stiffener and
to a heat treatment step in order to cleave the substrate at the layer of microcavities.
This process is, for example, used for the production of a thin silicon-on-insulator film, this thin film consisting of part of the substrate that has remained on the stiffener after cleavage.
With this type of process, the region where the implantation takes place is disturbed, thereby affecting the homogeneity of the thin film and its roughness at the surfaces located on either side of the cleavage surface. It is therefore necessary thereafter to carry out treatments to remove the defects, roughnesses, etc. These treatments consist, for example, in polishing the surfaces and/or healing the crystal structure by annealing. However, the greater the depthwise extent of the disturbed regions, the longer these treatments and the more affected the thickness homogeneity of the thin film and of the substrate remaining after cleavage.
Moreover, it is known that it is possible to use the micropores in the porous silicon to produce electroluminescent or photoluminescent components. However, these micropores obtained by electrochemical means extend over a great depth and are large in size, which may limit the quality of the luminescent phenomena that it is desired to exploit. It would therefore be advantageous for there to be semiconductor substrates having cavities the dimensions of which are better controlled and which extend over a better-localized depth so as to produce light-emitting components exhibiting better behavior.
One objective of the present invention is to propose a process allowing better control of the location, the formation and the growth of cavities.
This objective is achieved according to the invention by virtue of a process for treating substrates, especially semiconductor substrates, by atom implantation, for the purpose of creating cavities at a controlled depth in a substrate, characterized in that it comprises the steps consisting in:
implanting atoms into the substrate at a first depth, in order to obtain a first atom concentration at this first depth;
implanting atoms into the substrate at a second depth, different from the first, in order to obtain, at this second depth, a second atom concentration less than the first;
carrying out, on the substrate, a treatment capable of making at least some of the atoms implanted at the second depth migrate toward the first depth so as to preferably generate the cavities at the first depth.
When atoms are made to penetrate a substrate from a surface of this substrate, for example by implantation, they become distributed within the substrate. The concentration of these atoms in the substrate has a depth-dependent distribution profile which forms a peak with a maximum at a certain depth. For a given concentration, this peak has a width which is greater the greater the number of atoms that have penetrated the substrate. The presence of these atoms in the substrate disturbs the latter and the extent of the disturbed region is greater, for a given concentration, the wider the peak. However, certain special effects, such as embrittlement of the substrate sufficient to be able to cleave it or the formation of microcavities in order to obtain a luminescent effect, etc., require a high concentration of implanted atoms in the region where it is desired to obtain the sought effect. It is in this type of case that the process according to the invention is particularly beneficial since it makes it possible to limit the depthwise extent of the disturbed region while still achieving the concentration necessary for obtaining the desired effects.
The process according to the invention therefore consists of a step of implanting atoms into the substrate in order to obtain a first atom concentration forming a first peak with a first width and a first maximum located at a first depth. By limiting the number of atoms implanted at this step, it is possible, for a given concentration, to reduce the width of the concentration profile. This results in a reduction in the depthwise extent of the region most disturbed by the implanted atoms.
During another step of the process according to the invention, atoms are implanted into the substrate at a second depth, different from the first, in order to obtain, at this second depth, a second atom concentration less than the first. Thus, a reservoir of implanted atoms, of structural perturbations such as defects or vacancies generated by the implanted atoms, chemical species linked to implanted atoms, etc. is formed, it then being possible for these to migrate or be transferred to the vicinity of the first depth. The number of atoms implanted into this reservoir is adapted so that the sum of the atoms implanted at the first depth and implanted at the second depth is sufficient to produce cavities at the first depth after a treatment capable of making at least some of the atoms implanted at the second depth migrate toward the first. It is understood that this treatment may also transfer, from the second depth to the first depth, structural perturbations such as defects or vacancies generated by the implanted atoms, and chemical species linked to the implanted atoms (Vxe2x80x94H4). We will consequently use the generic expression xe2x80x9cspecies linked to the implanted atomsxe2x80x9d to mean all of these perturbations and of these chemical species linked to the implanted atoms.
With the process according to the invention, it is therefore possible to reduce the width of the peak produced at the first depth, by implanting fewer atoms into the substrate during the corresponding implantation step, but to obtain the number of atoms necessary to obtain the desired effect, in the vicinity of the first depth by the supply of the implanted atoms, and of the various species linked to the implanted atoms, which have migrated from the reservoir.
Further aspects, objects and advantages of the present invention will appear on reading the detailed description which follows.